Liquid crystals are used in electro-optical structural elements. Depending on the positive or negative character and the amount of dielectric anisotropy and depending also on the initial orientation of the compound different electro-optical results may be obtained. M. Tobias, International Handbook of Liquid Crystal Displays 1975-76, Ovum Ltd., London 1976.
For practical use it is necessary that the compounds are chemically and thermally stable and that they possess sufficiently low melting points and sufficiently high clear points.
Some of the known electro-optical effects of the liquid crystals permit the reproduction in colors of figures, numerals and images. For this purpose there are used for instance liquid crystals mixed with dichroitic dyestuffs. In thin films there then occurs a change of color after turning on or turning off an electrical field (guest-host effect). However, because of insufficient solubility and light fastness of the dyestuffs these materials have not found general use.
It is accordingly prior art knowledge that the preferential orientation of nematic liquid crystals can be modified by means of an electric field. This modification of the orientation can be used for various light modulation effects, particularly for the above-referred to color reproduction of numbers or symbols and reproduction of images in color.
A prior art process is based on a procedure where a nematic liquid crystal having negative anisotropy of the dielectric constants is placed between two glass slides which are provided with a transparent electrically conductive coating to obtain a homeotropic orientation (N-layer). If this arrangement is placed between two crossed polarizers there appears the transmitted or reflected light in color after application of an electric voltage (DAP effect): M. F. Schiekel K. Fahrenschon: Applied Physics Letters 19 391 (1971).
Another prior art process is based on an orientation of a nematic liquid having positive anisotropy of the dielectric constants in a manner that the longitudinal axes of the molecule in the mean are aligned in a direction parallel to the surrounding electrodes (P-layer). If this p-layer is placed between crossed polarizers in a diagonal position and an electric voltage is applied to the electrodes the color of the transmitted or reflected light can be controlled by the voltage (DP effect) (H. Zaschke, H. Schubert, F. Kuschel, F. Dinger, D. Demus: Pat. No. 95 892 of the German Democratic Republic).
Both of these processes require the use of two polarizers which results in the loss of part of the incident light due to absorption. Particular technical difficulties arise in these processes because the thickness of the film must be kept constant within very narrow limits (about .+-.1.mu.). The interference colors which are observed are usually highly sensitive to changes of the voltage and temperature.
These drawbacks may partly be avoided by another prior art process. In that case there are used electro-optical cells with twisted nematic or cholesteric layers or films of a weak twist in order to obtain the color-modulation of the light. In these processes a suitable dyestuff in the form of a non-liquid crystal or of a nematic, smectic or cholesteric compound is added to the liquid crystal having a particularly high positive dielectric anisotropy. These cells between crossed polarizers result in color effects in the transmitted or reflected light and after application of an electric field in color extinction (F. Kuschel, D. Demus, G. Pelzl; economic patent of the German Democratic republic No. 116,116). This process permits to reduce substantially the dependency of the color on the thickness of the film and the applied voltage. However, there remains the shortcoming resulting from the use of two polarizing filters.
A process has also become known wherein colored light modulation is effected with one liquid crystal cell by using only one polarizer. This process which is based on the guest-host effect (G. H. Heilmeier, L. A. Zanoni: Applied Physics Letters 13, 91 (1968) employs a dyestuff with positive dichroism which dyestuff is dissolved in a nematic liquid. If the liquid crystal mixture has a positive dielectric anisotropy a P-layer is produced. This P-layer in polarized light appears in color if the direction of the transition moment of the incorporated dyestuff molecules coincides with the direction of vibration of the light. After application of an electric voltage a switch is observed from a colored appearance to a faintly colored or colorless appearance.
For reasons of visibility it is preferred to use a liquid crystal mixture which has a negative dielectric anisotropy and permits to employ an N-layer. The layer in the absence of a voltage appears colorless or faintly colored. It becomes more strongly colored if it is reoriented in uniform manner by means of an electric field.
The objective of an electro-optical device by which the electrically affected areas (figures, letters, etc.) have a strong coloration and exhibit a strong contrast against the environmental area which is outside the electrical field--this objective can be accomplished with this process either not at all or only in an imperfect manner. The reason for this is that the prior art nematic liquids used for structural elements and having a negative anisotropy required too high a voltage (10 volts and more).
A further shortcoming of this prior art process for color modulation of light is the fact that the employed dyes have frequently only a low solubility in the nematic liquids. In these cases there are then obtained only faint color contrasts. Besides, there is the danger that the dyestuff crystalizes out at low operating temperatures. The light fastness of the employed dyestuff is also inadequate and therefore the color contrasts will weaken with continuing operation.
It is therefore an object of the present invention to provide for nematic liquid crystals which have high chemical or thermal stability, a strong inherent color and a marked dichroism in the visible spectral range, have sufficiently high clear points together with sufficiently low melting temperatures and possess a good light fastness.